1. Field of the Invention
The field of art to which this invention pertains is terpolymer molding compositions made up of a chemically reacted mixture of vinyl aromatics, methyl methacrylate, and acrylonitrile or methacrylonitrile monomers.
2. Description of the Prior Art
Commercially available styrene-acrylonitrile copolymer resins are thought to have excellent chemical resistance and good clarity but their heat distortion properties preclude their use in the presence of steam or high temperature applications. Additionally, polycarbonate resins, which are clear and possess excellent heat distortion properties, are not resistant to a wide variety of chemicals especially those found in many households.
The present terpolymer combination of polymerized vinyl aromatic, methyl methacrylate, and acrylonitrile or methacrylonitrile monomer possess all three of these properties having excellent chemical resistance, exceptional clarity and the ability to withstand relatively high temperatures before distortion properties are affected. More particularly, when vinyl aromatic, methyl methacrylate and acrylonitrile or methacrylonitrile terpolymer product is produced, the ratio of the monomers must be varied so as to prevent the production of a terpolymer composition which would contain more than a few tenths of a weight percent of unreacted monomer. Such unreacted monomer has been found to degrade the high heat distortion properties of such terpolymer products. Additionally, the ratios of the three monomer precursors must be varied so as to produce the required properties of the terpolymer, namely--clarity, solvent resistance and high heat distortion temperatures.
I have found that by reacting vinyl aromatic, methyl methacrylate and, acrylonitrile or methacrylonitrile monomers at polymerization conditions to produce a terpolymer having a weight ratio of vinyl aromatic to methyl methacrylate to acrylonitrile or methacrylonitrile units of from about 25 to 40: 50 to 65: 5 to 10 respectively and producing a finished product with less than about one-half of one percent by weight of unreacted monomer materials from the above described reactants, that a material possessing good clarity, resistance to deformation at high temperatures and good solvent resistance can be produced.